This invention relates to abrasive media, and aqueous slurries for chemical mechanical polishing and planarization of the interlayers of integrated circuits, memory disks, optical lenses, etc. More particularly, the invention relates to non-spherical nodular silica particles that when used as an abrasive media in chemical mechanical polishing provide a rapid rate of material removal, while producing surfaces which are free from unacceptable gouges and/or scratches. The invention also relates to stable aqueous dispersions of monodispersed non-spherical nodular silica particles which are essentially free of sodium, chlorine, lithium and other contaminants. Such slurries are particularly suitable for the chemical mechanical planarization of wafers and interlayers in semiconductor integrated circuit manufacturing.
The fabrication of integrated circuits includes a step of providing a substrate, such as a silicon or gallium arsenide wafer having a smooth uniform surface on one side thereof. Various layers, which are either conductive, insulating or semiconductor in nature are then formed on top of the smooth surface. In fabricating such circuits, it is also necessary to form conductive lines or similar structures above a previously formed layer. However, prior surface formations usually leave the top surface topography highly irregular, with bumps, areas of unequal elevation, troughs and other surface irregularities. As a result, global planarization is necessary to ensure adequate focal depth during subsequent photolithography, as well as removing any irregularities and surface imperfections during the various stages of the fabrication process. This planarization is generally accomplished by chemical mechanical polishing (CMP).
The process of chemical mechanical polishing is carried out using aqueous slurries of suitable chemicals and abrasive particles. The chemicals react with the surfaces being polished to form a reaction layer which is then abraded, or rubbed off, by the solid particles contained in the slurry. In many cases these particles are composed of silica. Heretofore, a type of silica known as fumed silica has been the principal source for these particles. Fumed silica is produced by the flame hydrolysis or burning oxidation of silane compounds such as SiCl4, HSiCl3 CH3SiCl3, CH3Si(OCH3)3, and the like. The ultimate, spherical, silica particles produced by the flame hydrolysis or burning oxidation of silane compounds are very small, on the order of 10 to 20 nanometers.
These very small particles aggregate while molten and fumed silica is recovered as tightly bound, or fused, chain-like agglomerates of these ultimate particles. The effective particle diameter of these non-spherical agglomerates after dispersion into a CMP slurry is on the order of 100 nm. Both during the preparation of dispersions from these particles, and in their employment for CMP, the chain-like agglomerates are randomly fractured to produce asperities which can plow or dig into the surface being polished, producing undesirable gouges and scratches. Scratches and gouges remaining in the surface of an interlayer of an integrated circuit after planarization are extremely undesirable because they adversely affect the reliability of subsequent layers, acting as sources of defects and sites for contamination.
Also, the plowing or digging action of fumed silica particles results in some of the particles remaining partially embedded in the polished surface at the conclusion of the CMP process. Therefore, cleaning of the polished surface is difficult, often requiring vigorous mechanical brushing during the cleaning process. Particle contamination of the polished surface of a memory disc, for example, can produce read/write errors. CMP slurries produced with fumed silica have an additional drawback in that they contain traces of chlorine resulting from the silica particles having been produced from chlorosilanes. Chlorides are an especially undesirable contaminant in integrated circuit manufacture.
In efforts to overcome the scratching, gouging and cleaning problems associated with fumed silica particles, spherical silica particles have been employed in CMP slurries. Such spherical particles have been obtained by the well-known technique of subjecting an aqueous sodium or potassium silicate solution to ion-exchange to produce ultrafine silica particles which are subsequently grown in size by Oswalt ripening, or by the hydrolysis of ethyl silicate using the so-called Stober process. The Stober process is disclosed in an article xe2x80x9cControlled Growth of Monodispersed Silica Spheres in a Micron Range,xe2x80x9d by Stober et al published in the Journal of Colloid and Interface Sci. 26, 62-69 (1968).
While the use of such spherical particles reduces the scratching and cleaning problems associated with slurries made with fumed silica particles, the material removal rate of such slurries, for equal particle concentrations, is dramatically lower. This is associated, of course, with the fact that the coefficient of rolling friction is less than that for sliding friction, and the removal rate of the chemically reacted layer is a function of the frictional force between it and the abrasive particle. While this reduced removal rate can be mitigated to a degree by significantly increasing the particle concentration of the spherical particles in the CMP slurry, this is at the expense of additional material cost, cleaning cost and spent-slurry disposal cost. Also, the slurries made from sodium or potassium silicate have these alkali metals as a contaminant, which are particularly undesirable in the manufacture of integrated circuits.
The CMP slurries of this invention overcome the problems of previous known CMP slurries containing silica particles as the abrasive media in that they provide (i) removal rates comparable to, or better than, fumed silica slurries but with significantly less scratching and improved cleanability, (ii) higher material removal rates than CMP slurries containing spherical silica particles of similar size and particle concentration, and (iii) higher chemical purity. The particles of this invention can be produced from mixtures of an alkoxysilane or alkoxyalkylsilane, ammonia, water and alcohol by adjusting the ratio of ingredients, conditions of particle nucleation and growth so that the final particles are nodular, rather than spherical; the result of two or more spherical nuclei having coalesced to form a single particle during the growth process.
When used as an abrasive media such particles will slide without plowing or digging into the surface being polished, and because of their higher coefficient of friction with the surface being polished will produce higher material removal rates than spherical particles. Also, because of the exceptional levels of chemical purity that can be attained and maintained with the chemicals and process, CMP slurries made with the particles described can have chemical purities previously unattainable.
The present invention contemplates abrasive media and aqueous slurries of such media suitable for use in the chemical mechanical polishing and planarization of semiconductor wafers, the metallic and dielectric interlayers of integrated circuits, memory discs, optical lenses, etc. The particles comprising the abrasive media are preferably at least 50% by weight non-spherical nodular silica particles having maximum linear dimensions of from about 20 to 1000 nanometers (nm). The particles comprise coalesced nuclei which are characterized by continued growth after initial contact.
The slurries in accordance with the present invention comprise aqueous dispersions of monodispersed non-spherical nodular shaped silica particles with concentrations of particles between about 0.5 to about 50% by weight and preferably between about 3 to about 25% by weight. The invention also contemplates aqueous slurries of monodispersed non-spherical nodular silica particles with chemical additions that can render them suitable for the CMP of metals and metal/metallic compound interlayers, as well as oxide glasses.
The invention will now be described in connection with the following photomicrographs.